For the Kurdish-led forces, the sting was not that American troops were withdrawing from Syria, which they knew would happen eventually. It was that after five years of their fighting and dying alongside American troops in the battle against ISIS Mr. Trump pulled the plug so suddenly that they were ill-prepared for what came next.

“It was a stab in the back,” said Nesrin Abdullah, a spokeswoman for the Kurdish women’s militia. “The Americans kept saying they would not allow the Turks to enter, but in the end that’s what happened.”

Part of the problem was that American officials sent conflicting messages about how long the United States would stay in Syria and what it was doing there.

Obama administration officials told their Kurdish allies that the partnership would last through the defeat of ISIS, but that the United States would help them play a role in Syria’s future. That message grew even more muddled over the last year, as Mr. Trump vowed to withdraw American troops while other officials in his administration said they would stay until Iran had left the country and there was a political solution in Damascus.